FM Global
Property Loss Prevention Data Sheets
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July 2014
Interim Revision January 2021
Page 1 of 28
ROOF-MOUNTED SOLAR PHOTOVOLTAIC PANELS
Table of Contents
Page
1.0 SCOPE ..................................................................................................................................................... 3
1.1 Changes ............................................................................................................................................ 3
1.2 Hazards ............................................................................................................................................ 3
1.2.1 Natural Hazards ....................................................................................................................... 3
1.2.2 Fire Exposure .......................................................................................................................... 3
2.0 RECOMMENDATIONS ............................................................................................................................. 4
2.1 Construction and Location ................................................................................................................. 4
2.1.1 Wind ........................................................................................................................................ 4
2.1.2 Fire Exposure and Classification ............................................................................................. 8
2.1.3 Gravity Loads and Roof Drainage .......................................................................................... 9
2.1.4 Hail ........................................................................................................................................... 9
2.1.5 Earthquake .............................................................................................................................. 9
2.2 Electrical .......................................................................................................................................... 10
2.3 Operation and Maintenance ............................................................................................................ 12
2.4 Human Element ............................................................................................................................... 12
3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................... 12
3.1 Basic Operation of PV Systems ...................................................................................................... 12
3.1.1 Earthquake Concerns ............................................................................................................ 12
3.2 Wind Resistance ............................................................................................................................. 13
3.2.1 Boundary Layer Wind Tunnel (BLWT) Testing and Ballasted PV Systems .......................... 13
3.2.2 PV Systems Fastened to Standing Seam Roofs (SSR) ....................................................... 15
3.2.3 Effective Wind Area .............................................................................................................. 17
3.2.4 Avoiding Roof Aggregate ....................................................................................................... 18
3.3 Fires and Electrical Ignition Sources ............................................................................................... 18
3.3.1 Ground Fault Protection ........................................................................................................ 18
3.3.2 Preventing Fires from DC Ground Fault in PV Arrays ......................................................... 19
3.4 Exterior Fire Spread in Roof-Mounted PV Arrays ........................................................................... 19
3.5 Reserved for Future Use ................................................................................................................. 19
3.6 Hail Resistance ................................................................................................................................ 19
3.7 Flexible PV Installations .................................................................................................................. 20
3.8 Information Needed for FM Global Plan Review ............................................................................ 20
4.0 REFERENCES ....................................................................................................................................... 20
4.1 FM Global ....................................................................................................................................... 20
4.2 Other ................................................................................................................................................ 21
4.3 Bibliography ..................................................................................................................................... 21
APPENDIX A GLOSSARY OF TERMS ...................................................................................................... 22
APPENDIX B DOCUMENT REVISION HISTORY ...................................................................................... 24
APPENDIX C SAMPLE PROBLEM: PV MODULES PARALLEL TO ROOF ........................................... 24
C.1 Example .......................................................................................................................................... 24
C.2 Solution ........................................................................................................................................... 25
C.3 Summary ......................................................................................................................................... 26
C.4 Discussion ....................................................................................................................................... 28
List of Figures
Fig. 2.1.1.1. Wind deflectors provided on the high sides of panels in each row (closed array) .................. 5
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Figs. 2.1.1.9a and 2.1.1.9b. Examples of mechanical anchors used to secure equipment to the roof
deck or roof framing ......................................................................................... 6
Fig. 2.1.1.11a. Slotted pedestal ..................................................................................................................... 7
Fig. 2.1.1.11b. Flanged pedestal ................................................................................................................... 7
Fig. 2.1.1.12. Wind zones for sloped PV arrays on low-slope roofs per SEAOC-PV2, 2017 ...................... 8
Fig. 2.1.2.1. Recommended roof expansion joint detail ................................................................................ 9
Fig. 2.2.2a. Example of residual current measurements with auxiliary trip (CB = combiner box,
RCD = residual current disconnect, GFDI = ground fault detection and interruption) ............ 10
Fig. 2.2.2b. Example of electronic current sensing relay in ground circuit ................................................. 11
Fig. 2.2.2c. Example of module level power electronics (courtesy of the National Fire Protection
Association) ............................................................................................................................... 11
Fig. 3.2.1.1a. Mechanically fastened roof cover billowing when subjected to wind pressure .................... 14
Fig. 3.2.1.1b. Solar panels with steeper slopes or lacking wind deflectors will experience greater wind
effects .................................................................................................................................... 15
Fig. 3.2.1.1c. Equipment lacking anchorage to roof framing ...................................................................... 16
Fig. 3.2.2a. Solar panels secured to standing seam roofs using external seam clamps ........................... 16
Fig. 3.2.2b. Unacceptable arrangement: clamp missing from SSR rib below middle of outer panel edge . 17
Fig. C.1-1. Plan view of proposed layout for PV modules and clamps ...................................................... 25
Fig. C.3-1. Wind zones for low-slope roofs (<=7°) per ASCE 7-16 ............................................................ 27
Fig. C.3-2. Various wind zones for proposed PV array in the example ...................................................... 27
List of Tables
Table
Table
Table
Table
Table
Table
2.1.4.1. Minimum Hail Ratings for PV Modules .................................................................................... 9
3.2.3.2. Prescriptive EWA for Ballasted PV Arrays with Rigid Interconnecting Racking .................... 18
3.8. Coefficients for Velocity Pressure (KZ) ........................................................................................ 20
C.2-1. Values of GCp per ASCE 7-16 and Data Sheet 1-28 .............................................................. 26
C.3-2. Preliminary Wind Design Pressures ........................................................................................ 28
C.4-1. Final Wind Design Pressures ................................................................................................... 28
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1.0 SCOPE
This data sheet provides property loss prevention guidance related to fire and natural hazards for the design,
installation, and maintenance of all roof-mounted photovoltaic (PV) solar panels used to generate electrical
power.
This document does not address solar towers, roof-mounted solar-powered water heaters, PV carports, or
ground-mounted solar farms. For guidance on ground-mounted solar farms, see Data Sheet 7-106, GroundMounted Photovoltaic Solar Power.
1.1 Changes
January 2021. Interim Revision. Minor editorial changes were made.
1.2 Hazards
1.2.1 Natural Hazards
1.2.1.1 Windstorm
Inadequate windstorm resistance can result in varying degrees of damage to roof-mounted PV solar panels.
In a worst-case scenario, they could be dislodged, break, and become windborne debris that damages other
panels and roof covers, allowing water to damage the building interior and contents.
1.2.1.2 Hail
Exposure to hail exceeding that which the panels have been tested and Approved for is likely to damage
all the panels in the array(s).
1.2.1.3 Snow and Ponding
Excessive loads from snow and rainwater accumulations on a roof in conjunction with the weight of these
PV systems can damage or collapse a roof, particularly where the PV systems impede rainwater flow to
drains.
PV panels with greater slopes and heights will increase snow accumulations and collapse potential unless
the roof can support the extra load.
1.2.1.4 Earthquake
Seismic activity can cause lateral or vertical movement of the panels. This can cause broken glass, damaged
electrical components, and an increased potential for ignition.
1.2.2 Fire Exposure
1.2.2.1 Exterior Fire Exposure
Exterior fire exposure due to the ignition of combustible components of the roof assembly below the PV panels
(or from adjacent buildings, yard storage, wildland fires and bushfires) can damage PV panels.
PV systems’ wiring circuits, combiner boxes, and inverter and control equipment are subject to electrical
failure and subsequent fire. The panels themselves create heat that can ignite debris on the roof surface below
the panels. Numerous fires started by the PV electrical system have involved combustibles within the roofing
assembly and were adversely affected by re-radiation of heat from the rigid PV panels.
Some PV racking systems use plastic frames, which can add significant fuel loading to a roof fire. Also, while
the top surfaces of the panels are covered with glass, the undersides of the panels are typically laminated
with a weather-resistant, polymeric encapsulant (back-sheet) and combustible adhesives (such as ethylene
vinyl acetate or EVA). This will add fuel to a roof-level fire and accelerate lateral fire spread.
The lower the classification (ASTM E108) of the exterior fire exposure of the roof assembly (cover and
insulation, C or B vs. A), the greater tendency there is for fire spread.
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In extreme cases, if above-deck roof components have low melting temperatures, they can burn and flow;
if there is a lack of protection at roof expansion joints, an exterior roof fire could spread into the building and
cause extensive interior damage.
2.0 RECOMMENDATIONS
Use FM Approved equipment, materials, and services whenever they are applicable and available. For a
list of roof assemblies that are FM Approved, see RoofNav, an online resource of FM Approvals.
Where installations are proposed at FM Global client locations, submit plans, specifications, and calculations
to the local FM Global office for review and comment prior to ordering materials. For details on what
information is needed, see Section 3.8.
2.1 Construction and Location
2.1.1 Wind
2.1.1.1 Design all roof-mounted, rigid PV solar panels and their securement using basic wind pressures in
accordance with DS 1-28, Wind Design. Adhere to the following recommendations except where noted
otherwise:
A. Use the design wind speeds as noted in Data Sheet 1-28. Do not further reduce the design wind speed
to that of a lower MRI based on assumptions regarding the expected lifespan of the arrays.
B. Use Exposure C in non-coastal areas unless all conditions for Exposure B are met as outlined in DS
1-28. Use Exposure D where needed per DS 1-28.
C. Use the topographic factor (KZT) as determined using ASCE 7 or DS 1-8. For locations with relatively
flat terrain (<6° or 10% ground slope), KZT can be assumed to be 1.0.
D. Use rigid PV solar panels and roof assemblies that are FM Approved together in accordance with
Approval Standard 4478, where available.
E. Multiply the basic wind pressure (qh) by the appropriate pressure coefficient considering whether the
array is ballasted or mechanically fastened, and the effective wind area using guidance in 2.1.1.2 through
2.1.1.6. The pressure coefficients used should reflect whether the PV arrays are open or closed (use wind
deflectors). See Figure 2.1.1.1.
A load factor (safety factor) of 1.6 times the wind load may be used for ballasted designs. For ultimate design
loads for mechanical anchors, use a minimum 2.0 factor.
2.1.1.2 Design wind pressure resistance for PV arrays that are parallel to the surface of low-slope roofs (≤7°)
and whose top edge is within 10 in. (254 mm) of the roof surface using pressure coefficients for low-slope
roofs per Data Sheet 1-28. An air equalization factor may be applied in accordance with SEAOC PV 2 (2017),
depending on the exact distance between the roof surface and top of the PV modules, as well as the gap
between modules in both directions. For an example, see Appendix C.
2.1.1.3 Determine the wind pressure resistance needed for ballasted or anchored roof-mounted PV panels
using one of the following options:
A. Provide wind resistance based on prescriptive calculation methods provided in SEAOC PV2 2017 (see
Section 4.2).
B. Provide wind resistance based on boundary layer wind tunnel (BLWT) data per ASCE 49 (or equivalent
international standard). Organizations that are qualified to conduct BLWT tests are noted in Section 3.2.1.
1. Have a qualified third party conduct a review of the BLWT test report.
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Fig. 2.1.1.1. Wind deflectors provided on the high sides of panels in each row (closed array)
2. Do not use computational fluid dynamics modeling as the primary substantiation for the design of
wind resistance.
3. For large installations (≥10,000 modules), have a qualified third party review the design for the
following:
a. The correct interpretation and application of BLWT data (see Section 3.2.1) at the specific site
b. The racking system structure to verify the adequacy of the effective wind area (EWA) at the specific
site
2.1.1.4 Install rigid PV solar panels over metal standing seam roofs (SSR) using external seam clamps (ESC)
that are FM Approved, where available, and properly fit the specific standing seam rib type at each seam.
Torque clamps and intermittently inspect for continued tightness in accordance with the manufacturer’s
instructions.
Install ESC at every roof deck seam, otherwise the installation is not acceptable and does not follow the
wind load path as designed by the SSR manufacturer. For new buildings, use SSRs that are FM Approved
in accordance with Approval Standard 4471, as specified in RoofNav, and installed in accordance with Data
Sheet 1-31, Panel Roof Systems. When installed over existing SSRs, the adequacy of the roof should first be
confirmed. Secure clamps as close as practical to the internal seam clips securing the SSR panels to purlins.
Ensure design wind loads are in accordance with the recommendations in Section 2.1.1.1, 2.1.1.2, or
2.1.1.3, as applicable.
2.1.1.5 Install ballasted rigid PV roof-mounted solar panels roofs with a maximum roof slope of 1/2 in. per
ft (2.4°). A higher slope is not recommended for ballasted PV panels as it will decrease frictional resistance
to wind forces and increase sliding forces from gravity loads, weakening wind resistance. Use a combined
weight of solar panels, associated hardware, and additional concrete paver blocks as needed to meet wind
loads per Sections 2.1.1.1, 2.1.1.2, or 2.1.1.3 as applicable.
Use a coefficient of static friction (µ) of 0.4 in the design unless a higher value can be justified (the lesser
of the wet or dry value) based on the materials used and testing in accordance with ASTM D1894 (or
equivalent standard outside the United States).
2.1.1.6 Install ballasted, rigid roof-mounted PV panels over fully adhered roof covers.
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There is no consensus wind design method for installing ballasted PV arrays over mechanically fastened
single-ply roof covers.
2.1.1.7 Use concrete paver blocks for ballasted PV panels that meet specifications in ASTM C1491 and are
tested in accordance with ASTM C1262 (does not include pass/fail criteria) for exposure to freeze-thaw
cycles. The cumulative weight loss measured in the test should not exceed 5% of the initial weight of the
specimen. (Use comparable standards outside the United States.)
2.1.1.8 Do not install PV modules on roofs with aggregate, including pea gravel or larger stone ballast.
2.1.1.9 Anchor all related equipment, such as combiner/junction boxes and conduits, to the roof deck or roof
structural members (or inverters to concrete foundations) as required to provide proper anchorage against
expected loads (see Figures 2.1.1.9a, 2.1.1.9b, and 3.2.1.1c). Use mechanical anchors that can be connected
to the equipment and to the roof deck or roof framing. The dead weight and resulting frictional resistance
for most equipment is not sufficient to resist wind uplift and lateral wind loads.
Figs. 2.1.1.9a and 2.1.1.9b. Examples of mechanical anchors used to secure equipment to the roof deck or roof framing
2.1.1.10 During installation, complete all required steps for the securement of PV panels before the end of
each shift. This includes the mechanical connection to previously installed panels and any needed additional
ballast.
2.1.1.11 Provide a positive method of securement between concrete paver blocks and pedestals or paver
trays. This could include slotted or flanged pedestals or paver trays (see Figures 2.1.1.11a and b).
2.1.1.12 Where wind loads are too high to make ballasting practical throughout, hybrid systems (ballast and
anchors) can be used in accordance with one of the following:
A. Provide mechanical anchors for the entire array. Design the anchors to resist loads considering their
effective wind area and providing a safety factor of 2.0 based on allowable stress design (ASD); or 1.25
times the Load and Resistance Factor Design (LRFD, or Strength Design) load. Have a third-party review
performed by a licensed structural engineer.
B. Provide mechanical anchors (designed per item A) for all modules within perimeter zones of the array
and provide ballast for interior zones, if roof strength is adequate. Have a third-party review performed
by a licensed structural engineer.
C. Provide additional setback distance between the roof edges and the edges of the array so that the
arrays are in wind zones with lower pressures, and anchor (designed per item A) or ballast accordingly.
For example, arrays with a setback distance ≥2H (where H = roof height) are in Zone 1 or 1’, where wind
pressures are much less than in Zone 3 or 2 (see Figure 2.1.1.12).
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Fig. 2.1.1.11a. Slotted pedestal
Fig. 2.1.1.11b. Flanged pedestal
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Fig. 2.1.1.12. Wind zones for sloped PV arrays on low-slope roofs per SEAOC-PV2, 2017
Zone 3 (Red): Building Corner, 2h x 2h
Zone 2 (Yellow): Building Perimeter, 2h wide between corners
Zone 1 (White): Distance greater than 2h inward from Zones 3 and 2
Zone 1’ (not illustrated above): Exists for relatively wide buildings only >10h. Beyond distance of 5h from
building edges.
2.1.2 Fire Exposure and Classification
2.1.2.1 Provide noncombustible, compressible insulation (such as mineral wool) within roof expansion joints
or around other roof penetrations when new PV installations are to be installed on new or existing roof covers.
See Figure 2.1.2.1.
•
Use steel (Galvalume or
stainless) expansion joint
cover
•
Compressible mineral
wool insulation
•
Wood nailers both sides
•
NC insulation or cover
board below cover
Fig. 2.1.2.1. Recommended roof expansion joint detail
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2.1.2.2 When new roofs are to be installed, do one of the following (in order of preference) to minimize exterior
roof fire spread:
A. Use PV panels that are FM Approved with the specific roof system per Approval Standard 4478.
B. Use PV modules with glass back sheets, and install roof assemblies that are FM Approved Class 1A
rated panel roofs with a metal top surface.
C. Use PV modules with non-glass back sheets and install roof assemblies that are FM Approved Class
1A rated panel roofs with a metal top surface.
D. Use PV modules with glass back sheets and install roof assemblies that are FM Approved Class A
rated single-ply membrane systems that use a layer of FM Approved gypsum board (min. 1/4 in. [6 mm]),
mineral wool, or expanded glass (coverboard or insulation) directly below the cover.
2.1.2.3 Do not use PV panel systems that contain thermoplastic foam, such as extruded or expanded
polystyrene. The roof assembly should maintain a Class 1 or noncombustible fire rating for underside fire
exposure.
2.1.2.4 Do not install PV arrays within 50 ft (15 m) of maximum foreseeable loss (MFL) walls (see DS 1-42,
MFL Limiting Factors).
2.1.2.5 Provide sufficient aisle spaces between adjacent PV arrays, adjacent rooftop equipment or
penetrations, and between PV panels and expansion or control joints on each side. Submit the proposed
layout to the fire service for review and acceptance. Minimum 4 ft (1.2 m) wide aisles at a maximum of 150
ft (46 m) in each direction are recommended; some jurisdictions may require wider or more frequent aisles.
2.1.3 Gravity Loads and Roof Drainage
2.1.3.1 Install PV systems on roofs with minimum slopes of 1/4 in. per ft (1.2°), but not greater than that
noted in Section 2.1.1.5.
2.1.3.2 Design the PV modules and the roof supporting them to resist design snow loads, including potential
drifting, in accordance with DS 1-54. FM Approved PV modules are evaluated for gravity load resistance.
2.1.3.3 When PV systems are proposed for existing roofs, ensure the dead weight of the proposed PV system
does not reduce the roof resistance recommended in DS 1-54 for snow, rain, and other live loads to below
acceptable levels. Consider 2 to 3 psf (0.10 to 0.14 kPa) for the PV modules and hardware plus additional
recommended ballast weight.
2.1.3.4 Ensure the path for rainwater flowing to roof drains is unobstructed for all PV arrays. Analyze in
accordance with DS 1-54, Roof Loads for New Construction.
2.1.4 Hail
2.1.4.1 Use PV modules that have hail ratings (established in accordance with FM Approval Standard 4478
or 4476) as recommended for hail-prone regions as defined by DS 1-34. See Table 2.1.4.1.
Table 2.1.4.1. Minimum Hail Ratings for PV Modules
Hail-Prone Region
Moderate hail
Severe hail
Very severe hail
Rigid FM 4478
Class 2
Class 3 or Class 4
Not available
Flexible FM 4476
MH
SH
Not available
2.1.5 Earthquake
2.1.5.1 Design rigid PV solar panels located in seismic zones 50 through 500 years to prevent lateral
movement during a design seismic event. (For determination of seismic zones and other details, see DS 1-2.)
This could be done by providing anchorage to the roof deck or framing around the entire perimeter of each
array. The design of the anchors should consider not only the strength of the anchors but the transfer of
loads directly to secondary roof framing or through the deck and the deck securement into the secondary
roof framing.
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Use bolted or other positive fastening methods as required by Chapter 13 of ASCE 7. Do not consider frictional
resistance dependent on gravity. Use PV modules that have been FM Approved in accordance with Approval
Standard 4478, where available.
See Section 3.1.1 for additional information.
2.2 Electrical
2.2.1 Install new PV electrical energy systems, including the array circuit(s), inverter(s), and controller(s)
for these systems, in accordance with Article 690 of the 2017 version of NFPA 70, National Electrical Code
(or equivalent international standard). Provide Module Level Power Electronics (such as DC optimizers and
microinverters) that sense and isolate faults and deenergize the array at the module level, and alarm such
faults. The system should report the alarm condition to remote network monitoring software, enabling rapid
shutdown of PV systems on buildings as defined in NEC 2017. See Figure 2.2.2C.
For more information, see Section 3.3.
RCD current
transducers
CB1
Inverter
+
CB 2
Relay
circuit
Aux. trip
AC
CB 3
CB 4
GFP
5A
CB 5
Fig. 2.2.2a. Example of residual current measurements with auxiliary trip (CB = combiner box, RCD = residual current
disconnect, GFDI = ground fault detection and interruption)
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Combiner box
Inverter
+ 600 Vdc
+
Aux. trip
0 Vdc
GFP
Electronic
sense
relay
Fig. 2.2.2b. Example of electronic current sensing relay in ground circuit
Fig. 2.2.2c. Example of module level power electronics (courtesy of the National Fire Protection Association)
2.2.2 Do not install electrical wiring within the rib opening of steel decking or otherwise within the plane of
the above-deck components. Besides serving as a possible ignition source, it would also inhibit access for
maintenance and repair and be subject to damage from mechanical fasteners used to secure above-deck
roof components.
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2.2.3 Ensure adequate provision is made for expansion and contraction due to extreme temperature
fluctuations during the year. This includes wiring, as well as the interface between the PV panels and the
roof cover.
2.2.4 Design and install interior cables and bus-bars in accordance with DS 5-31.
2.2.5 Use rigid PV panels that meet electrical performance criteria per IEC/EN 61215-1, 61215-1-1, and
61215-2.
2.2.6 Use rigid PV panels that comply with criteria for electrical safety per IEC/EN 61730-2, Photovoltaic
(PV) Module Safety Qualifications, Part 2: Requirements for Testing, or ANSI/UL 1703, Flat Plate Photovoltaic
Modules and Panels.
2.3 Operation and Maintenance
2.3.1 Check all equipment for damage or required maintenance after seismic or severe weather events,
including windstorm, lightning, hail, and snow storms.
2.3.2 Inspect the sealing of roof penetrations for water-tightness annually, and repair or replace as needed.
2.3.3 Have routine inspection, testing, and maintenance of the PV arrays and related equipment performed
by qualified personnel and in accordance with the manufacturers’ guidelines.
2.4 Human Element
2.4.1 Arrange pre-fire planning with the fire service. Ensure they are familiar with ground access, stairs to
the roof, PV array aisles, the location of combiner boxes and inverters, and all related fuses and disconnects.
3.0 SUPPORT FOR RECOMMENDATIONS
3.1 Basic Operation of PV Systems
Rigid PV solar panels are made of semiconductors in the form of individual silicon cells wired in series, and
usually protected above by tempered glass and on the bottom by a polymeric encapsulant (back-sheet).
Back-sheets are laminated in up to 3 layers and can consist of almost any combination of ethylene vinyl
acetate (EVA), polyethylene terephthalate (PET), Kynar, or Tedlar. An anti-reflective coating is provided on
the top surface. Modules are linked together in series to form strings, and then individual strings are connected
within a combiner box to form an array. The modules within the array convert energy from sunlight into direct
current (DC) electrical power. This power can be stored as DC, but more commonly it is converted to AC
using an inverter, and then fed into a large electrical grid, or in some cases used directly on-site. Usually one
or more arrays/combiner boxes are connected to an inverter when the electric power is converted from DC
to AC.
Common sites for PV panels are roofs of warehouses and other facilities that do not require extensive rooftop
equipment that would shadow the PV panels. Aisles are often provided within or between arrays to allow
access for maintenance of rooftop equipment and manual firefighting, as well as to prevent the panels being
shadowed by other equipment, higher roofs, or other obstructions to sunlight. For additional information on
rigid PV panels, see DS 7-106.
3.1.1 Earthquake Concerns
Seismic load concerns are somewhat different from wind load concerns. In seismic design, greater emphasis
is placed on lateral forces. Some lateral movement may be tolerable from a life safety perspective, and
therefore acceptable per building codes. Significant lateral movement can result in considerable damage to
PV modules.
PV arrays may be provided with sufficient ballast to resist vertical wind loads, which are a greater concern
with wind than lateral loads. While frictional resistance caused by gravity loads (combined weight of PV
modules, racking, and added ballast) can help resist lateral forces due to seismic acceleration, that lateral
force is a function of dead weight. Thus, increasing the ballast beyond what is needed for wind design also
increases lateral seismic loads.
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Lateral loads distribute more efficiently within a PV array, whereas vertical load distribution is very localized.
Providing mechanical anchorage around the entire perimeter of a PV array is a preferred method to reduce
PV module damage during seismic events.
3.2 Wind Resistance
3.2.1 Boundary Layer Wind Tunnel (BLWT) Testing and Ballasted PV Systems
Testing in a boundary layer wind tunnel (BLWT) is conducted to determine wind loads and resistance for
roof-mounted PV panels. It is important that the scaled models used to replicate the proposed roof-mounted
panels be as representative as possible, particularly with ballasted arrays. This includes the sizes of individual
panels, the weights of the panels and ballast, the PV panel slope (see Figure 3.2.1.1b), the coefficient of
friction (µ) between the roof surface and the underside of the panel pedestals or paver trays, and the size
of the array. Tests should replicate the minimum array size to be used, regarding the number of interconnected
panels within a given array and the minimum number of panels within a row or column.
To allow the test data to be used for a variety of combinations of roof cover types and pedestal pads/paver
trays, separate testing may be needed to quantify the coefficient of friction between the two surfaces. Testing
should reflect any slip sheets that may be used. Since movement of any panel defines failure, the use of the
static coefficient of friction may be used in lieu of the dynamic value. While often the wet coefficient of friction
yields a lower value, test data reflects that in some cases the dry value is lower.
Testing needs to be conducted in a boundary layer wind tunnel (BLWT) rather than an aerospace wind tunnel
(AWT). While there are some similarities between the two types, the BLWT simulates wind flow toward a
building by providing obstructions between the entrance of the wind into the tunnel and the scaled building
model. Typically, an open terrain or Exposure C is simulated. The simulated building is often a flat rigid object.
This allows the wind to hit the wall of the model, flow over it, and create turbulence and vortices that cause
higher uplift pressures above the roof, particularly at the perimeter and corner areas. Such a realistic effect
is not provided when using an aerospace wind tunnel.
Even in a BLWT, internal building pressure effects and potential vertical movement of the roof cover are not
simulated. The building models used in a BLWT test are very rigid and do not represent the behavior of a
mechanically fastened roof cover (see Figure 3.2.1.1a), which may billow when exposed to wind pressure.
Such vertical movement of the roof cover can increase the drag and lift coefficients for the PV modules, and
can make the results of the BLWT invalid. The results of the BLWT test are more applicable to a fully adhered
roof cover. PV panels used over mechanically fastened roof cover should be mechanically fastened.
While there are numerous aerospace wind tunnels, a limited number of BLWTs exist. The following locations
have BLWTs:
• Colorado State University (CSU)
• Western University (formerly the University of Western Ontario or UWO), Ontario, Canada
• Cermak, Peterka and Peterson (CPP) in Colorado and Australia
• Rowan, Williams, Davies and Irwin, Inc. (RWDI), Canada
• I.F.I. Institute, Germany
• Force Technology, Europe
• University of California, Davis
• University of Maryland
• University of Minnesota
• Concordia University, Montreal, Quebec, Canada
• Peutz Ltd, Mook, The Netherlalnds
A. Experimental wind load estimates on roof-mounted solar panels can be inaccurate for the following
reasons:
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1. The experiments were conducted without considering the effect of the building on the solar panels.
This includes experiments that were conducted in an aerospace wind tunnel, which is used for testing cars
and aircraft. These types of wind tunnels produce smooth wind at a constant speed, and at very low
turbulence intensity (<= 0.5%). To study the wind load on roof-mounted solar panels, experiments must
be conducted in a BLWT, where the wind is turbulent and gusty with high turbulence intensity (≤10%). The
wind tunnel experiments also must be conducted in accordance with the ASCE’s Wind Tunnel Studies
of Buildings and Other Structures.
2. The experiments were conducted only for a single wind direction. Just like the roof itself, the tilted solar
panels can experience substantial wind loads from cornering winds.
B. Wind load estimates obtained using only computational fluid dynamics simulations on roof-mounted solar
panels are not recommended by ASCE and may be inaccurate for the following reasons:
1. The simulations were performed without considering the effect of the building on the solar panels.
2. Validation of the computational fluid dynamics simulations with existing literature or with BLWT
experiments were not performed.
3.2.1.1 Increased Ballast or Securement Around Openings and Aisle Spaces
Often, there will be aisle spaces around other roof-mounted equipment, or between arrays that provide fire
fighter or maintenance access, that break the continuity of the interconnection between panels. This reduces
the wind load distribution, as well as the shielding affect against wind that the outer panels in the array provide
for those panels farther in from the aisles. To account for this, additional ballast or securement (typically 50%
more) should be provided for the panels immediately around the openings.
Fig. 3.2.1.1a. Mechanically fastened roof cover billowing when subjected to wind pressure
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Fig. 3.2.1.1b. Solar panels with steeper slopes or lacking wind deflectors will experience greater wind effects
3.2.2 PV Systems Fastened to Standing Seam Roofs (SSR)
Rigid PV panels can be mechanically fastened to SSRs and can be FM Approved in accordance with Approval
Standard 4478. For more information on SSRs, see DS 1-31. SSR panels are seamed to the internal clips,
which are pre-fastened at each deck rib to each steel purlin or a continuous substrate. The wind design for
SSR assumes the wind load is distributed evenly to each internal clip. An external seam clamp, like those used
to enhance the wind resistance of SSRs, is used to connect PV panels to the SSR deck ribs (see Figures
3.2.2a and 3.2.2b). These clamps do not penetrate the seam. One clamp should be provided at each standing
seam rib at the down-slope and up-slope edges of the PV panels. Otherwise the wind load transferred from
the PV modules to the internal SSR clip and screws securing the clip to the top flanges purlin may be
overloaded. The spacing between clamps may vary from about 3 to 10 ft2 (0.3 to 1.0 m2) per clamp, depending
on the SSR rib spacing and the distance between internal clips along the deck seams. It is important that
the individual clamp be designed to fit the specific seam of the SSR. For an example problem, see Appendix
C.
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Fig. 3.2.1.1c. Equipment lacking anchorage to roof framing
Fig. 3.2.2a. Solar panels secured to standing seam roofs using external seam clamps
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Fig. 3.2.2b. Unacceptable arrangement: clamp missing from SSR rib below middle of outer panel edge
3.2.3 Effective Wind Area
The effective wind area (EWA) reflects the area of a given component of an assembly to which the wind
load is distributed. For a fastener, the EWA can be assumed to be the area supported by the fastener.
For ballasted PV arrays, determining the EWA for wind uplift can be complicated. It is critical that the EWA
be accurate. As the EWA increases, the wind pressure coefficient decreases. Using an unrealistically large
EWA in the design calculations will result in wind resistance that is too low. The EWA for a ballasted array
varies depending on the location of the module within the array (i.e., corner, edge, or interior) as well as
the rigidity of the hardware or racking that connects the modules. The EWA can be determined using one
of the following methods:
A. Finite Element Analysis (FEA) of the hardware or racking assembly
B. A full scale vertical load distribution (VLD) test
C. For modules connected with rigid racking members, but for which the information above is not available,
use guidelines in Table 3.2.3.3.
3.2.3.1 Finite Element Analysis (FEA)
Finite element analysis can be used by the structural design engineer to establish structural capacity curves
for the ballasted PV array for a range of applicable ballast weights (for which the PV array is typically
designed). Tributary area can be determined from the intersection of the structural capacity curve and the
design wind load (which can be calculated using SEAOC PV 2). Effective wind area is assumed to be the same
as the tributary area. The following steps explain the procedure to calculate the structural capacity curve.
Step 1: Identify governing loading areas (i.e. corner, edge and interior area of the PV array).
Step 2: For each loading area, define governing loading scenarios that can result in the least resistance or
structural capacity (i.e., one panel loaded, two panels loaded, three panels loaded etc.).
Step 3: For each loading area, perform nonlinear finite element analysis of the PV array for each loading
scenario and ballast weight, considering uniform wind load on each panel and applicable boundary conditions
and materials of the PV array as built in practice. During the analysis, increase the wind load until the system
reaches any failure criterion such as permanent deformation or maximum uplift displacement of a portion
of the PV array.
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Step 4: For each loading area, plot the wind loads (from the analysis) of different loading scenarios (number
of panels loaded) for each ballast weight. Repeat this and add the curves for all other ballast weights to the
same plot. The resulting plot is the capacity curve for the applicable loading area.
Step 5: Repeat Steps 2 to 5 and obtain the structural capacity curves for all the governing loading areas.
The above information is intended for use by the structural design engineer, who should verify that this
procedure was followed. Detailed review by field engineering is not practical.
3.2.3.2 Prescriptive EWA for Ballasted PV Arrays
In the absence of FEA or VLD test data to determine the EWA, use Table 3.2.3.3 for arrays with rigid
interconnected racking.
Table 3.2.3.2. Prescriptive EWA for Ballasted PV Arrays with Rigid Interconnecting Racking
Location of Module within Array
Corner
Edge
Interior
Number of Modules in EWA
4
6
9
3.2.4 Avoiding Roof Aggregate
The presence of roof aggregate where there are roof-mounted PV modules could result in windborne debris
damage to the PV panels. If ballasted PV pedestals or paver trays are installed directly on top of roofing
aggregate, it can adversely affect the arrays’ resistance to sliding. Roof cover ballast that is continuous over
the entire roof cover and consists of concrete paver blocks designed in accordance with DS 1-29, Roof Deck
Securement and Above-Deck Roof Components, are acceptable if a sufficient weight of concrete paver
blocks is provided above the solar panel pedestals or paver trays to provide the needed wind resistance for
the solar panels.
3.3 Fires and Electrical Ignition Sources
3.3.1 Ground Fault Protection
Numerous fires have started in U.S. installations of roof-mounted PV arrays due to inadequate ground fault
protection. Such installations in the United States typically include conductors that are intentionally grounded,
but have ground fault detection designed for ungrounded conductor faults. This design is based on
conservative assumptions of leakage current to avoid nuisance trips. However, the present ground fault
detection uses fuses that are not sensitive enough, resulting in undetected ground faults. Such systems have
become more prevalent in recent years and, as they continue to age, the frequency of such fires could
increase.
Fires of electrical origin are common in roof-mounted solar arrays. There are sufficient combustibles present
in the form of roof coverings and insulation, which are more likely to become ignited with the PV system
there. Also, the redirection of flames and re-radiation of heat by the PV panels from a roof fire tend to create
more fire spread than if the panels were not there. Following the electrical guidance in this document will
reduce, but not eliminate, the potential for a fire.
The goal is to identify an initial ground fault prior to the second ground fault. Recent losses have shown that
traditional ground fault protection (GFP) using fuses per older versions of Article 690 of the NEC was not
sufficiently sensitive and allowed “blind spots” with an undetected initial ground fault. Given a second ground
fault, this can result in enough energy to start a roof-top fire.
Included in NEC 2017 is a requirement to reduce voltage within the array, also commonly referred to
“module-level rapid shutdown.” This is done to reduce voltage in the array during emergency situations or
general maintenance. The most widely implemented solution is using module-level rapid shutdown electronics
such as DC optimizers.
In addition to ground faults, module-level power electronics can provide arc fault circuit interruption (AFCI)
and monitoring for these conditions as well. Module-level power electronics do not rely on fuses for ground
fault protection.
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3.3.2 Preventing Fires from DC Ground Fault in PV Arrays
A ground fault in a PV array is an accidental electrical short circuit involving ground and one or more normally
designated current-carrying conductors. Ground faults in PV arrays are safety concerns because they may
generate DC arcs at the fault point on the ground fault path, damage surrounding insulation, and create fire
hazards. The risk of fire is escalated substantially if a second ground fault is developed. A DC ground fault
is common in PV systems and result from the following causes:
A. Insulation failure of cables (e.g., an animal chewing through cable insulation and causing a ground
fault)
B. Incidental short-circuit between the normal conductor and ground (e.g., a cable in a PV junction box
incidentally contacting a grounded conductor)
C. Ground faults within PV modules (e.g., a solar cell short-circuiting to grounded module frames due to
deteriorating encapsulation, impact damage, or water corrosion in the PV panel
D. Abraded wire insulation caused during installation or from thermal movement of the components
To properly protect PV arrays from ground fault damage and ensuing fire, NFPA 70, National Electrical Code,
Article 690.5(A), specifies the ground fault protection device (GFPD) or system must be capable of detecting
a ground-fault current, interrupting the flow of fault current, and providing an indication of the fault. Per past
industry experience, there are some cases in which the first ground fault could not be detected by the currently
designed GFPD (such as applying a fuse in the grounding electrode). A second ground fault made the fault
current flow in the array, leading to fire.
3.4 Exterior Fire Spread in Roof-Mounted PV Arrays
Where roof-mounted PV arrays are present, the risk of exterior fire spread is much greater than it would
be for the roof assembly alone. This would be the case even if the solar panels had no combustible
components. A typical fire scenario is the electrical wiring associated with the solar PV array causing ignition
of the roof assembly. The potential flame height is largely a function of the type of roof cover and insulation
immediately below the array. While the presence of solar panels may affect combustion air being drawn to
the fire, it otherwise does not reduce, but redirects the flames from the roof fire.
Solar panels containing foam plastic are not common, and will not alter the noncombustible (underside fire
exposure) rating of concrete, gypsum, lightweight insulating concrete decks, or Class 1 steel deck assemblies
with a thermal barrier, such as gypsum board. If it is not obvious that the integrity of an existing Class 1 steel
deck has been maintained, assume it is Class 2 for the purposes of determining the need for sprinklers below
the deck, or the MFL. There may still be a concern about exterior fire exposure.
PV panels that contain no foam plastic will not alter the existing Class 1 underside fire exposure rating for
steel deck.
Components of more common types of rigid PV panels (such as plastic frames and back-sheets and
adhesives) can ignite and radiate heat back to the roof cover and insulation, resulting in much greater exterior
fire spread than would be expected with the roof assembly itself. Consequently, only specific roof assemblies
are acceptable regarding fire spread with roof-mounted PV panels present.
Aisle spaces between PV arrays can be used by the fire service to ventilate a fire within the building. The
fire service may also use manually operated mechanical exhaust fans to vent an occupancy fire. Aisles have
been used by the fire service in their efforts to limit fire spread across the top of the roof by cutting trenches
in the above-deck components down to the top of the roof deck, breaking the continuity of combustibles
within the above-deck roof components.
3.5 Reserved for Future Use
3.6 Hail Resistance
Hail resistance of rigid PV panels should be determined by ice ball testing in accordance with Approval
Standard 4478. Hail resistance of flexible PV panels should be determined by steel ball testing in accordance
with Approval Standard 4476.
Impact from hail larger than that for which the panels were successfully tested could cause severe damage
to the PV panels.
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3.7 Flexible PV Installations
Adhered, flexible solar panels have been FM Approved in the past. They were required to be adhered across
their entire underside. Flexible solar panels that are only secured around their edges will not uniformly
distribute the wind load to the roof cover they are adhered to.
3.8 Information Needed for FM Global Plan Review
The following information is needed for an FM Global plan review:
A. Will the existing roof cover remain? If yes, indicate the type of roof cover, type of insulation or cover
board immediately below the roof cover, and whether the roof cover is fully adhered to it. Provide a sketch
of the roof expansion joint, including the type of insulation within it, considering Recommendation 2.1.2.1.
B. If a new roof cover is proposed, submit complete details for the roof cover, insulation and cover boards,
securement methods, and expansion joint description considering Recommendations 2.1.2.1 and 2.1.2.2.
C. Submit the calculation for basic wind pressure (qh) including the design wind speed, related coefficients
for velocity pressure (see Table 3.8), directionality, topography, etc.
D. The submittal should verify that the wind pressure coefficients used are based on the effective wind
area (EWA, see Section 3.2.3) and the location of the modules within the array. The EWA for vertical load
distribution for ballasted PV arrays is typically limited to the area of a few modules. Usually the EWA will
be the area of 1 to 9 modules depending on the location of the module within the array (corner, edge,
or shielded/interior). PV racking that uses rigid framing will have larger EWAs than arrays that have less
rigidity.
Table 3.8. Coefficients for Velocity Pressure (KZ)
Height above ground level, z
ft
m
0-15
0-4.6
20
6.1
25
7.6
30
9.1
40
12.2
50
15.2
60
18
70
21.3
80
24.4
90
27.4
120
36.6
140
42.7
160
48.8
180
54.9
200
61
250
76.2
300
91.4
400
121.9
450
137.2
500
152.4
B
Exposure
C
D
0.57
0.62
0.76
0.70
0.76
0.81
0.85
0.89
0.93
0.96
1.04
1.09
1.13
1.17
1.20
1.28
1.35
1.47
1.52
1.56
0.85
0.90
0.94
0.98
1.04
1.09
1.13
1.17
1.21
1.24
1.31
1.36
1.39
1.43
1.46
1.53
1.59
1.69
1.73
1.77
1.03
1.08
1.12
1.16
1.22
1.27
1.31
1.34
1.38
1.40
1.48
1.52
1.55
1.58
1.61
1.68
1.73
1.82
1.86
1.89
4.0 REFERENCES
4.1 FM Global
Data
Data
Data
Data
Sheet
Sheet
Sheet
Sheet
1-2, Earthquakes
1-28, Wind Design
1-29, Roof Deck Securement and Above-Deck Roof Components
1-31, Panel Roof Systems
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Data
Data
Data
Data
Data
Data
Data
Data
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
Sheet
1-34, Hail Damage
1-42, MFL Limiting Factors
1-54, Design Loads for New Construction
5-11, Lightning Protection
5-19, Switchgear and Circuit Breakers
5-20, Electrical Testing
5-23, Emergency and Standby Power Generating Systems
7-106, Ground-Mounted Photovoltaic Solar Power
FM 4476, Approval Standard for Flexible Photovoltaic Modules
FM 4478, Approval Standard for Rigid Photovoltaic Modules, December 2016
ANSI/FM 4473, Test Standard for Impact Testing of Rigid Roofing Material by Impact Testing with Freezer
Ice Balls
Approval Guide, Building Materials section, an online resource of FM Approvals
RoofNav, an online resource of FM Approvals for roofing professionals
4.2 Other
American Society of Civil Engineers (ASCE). Minimum Design Loads for Buildings and Other Structures.
ASCE 7, 2016.
American Society of Civil Engineers (ASCE). Wind Tunnel Studies of Buildings and Other Structures. Manual
of Practice 67.
American Society of Civil Engineers (ASCE). Wind Tunnel Testing for Buildings and Other Structures. ASCE
49.
ASTM International. Standard Specification for Concrete Roof Pavers. ASTM C1491-11.
ASTM International. Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry-Cast Segmental
Retaining Wall Units and Related Concrete Units. ASTM C1262-10.
International Electrotechnical Commission (IEC). Grid Connected Photovoltaic Systems: Minimum
Requirements for System Documentation, Commissioning Tests and Inspection. IEC 62446.
International Electrotechnical Commission (IEC). Crystalline Silicon Terrestrial Photovoltaic (PV) Modules:
Design Qualification and Type Approval. IEC/EN 61215-1, 61215-1-1, and IEC/EN 61215-2.
International Electrotechnical Commission (IEC). Photovoltaic (PV) module safety qualifications, Part 2:
Requirements for Testing. IEC/EN 61730-2.
National Fire Protection Association (NFPA). National Electric Code. NFPA 70, 2017.
Structural Engineers Association of California (SEAOC). Structural Seismic Requirements and Commentary
for Rooftop Solar Photovoltaic Systems. SEAOC PV1-2012.
Structural Engineers Association of California (SEAOC). Wind Design for Low-Profile Solar Photovoltaic
Arrays on Flat Roofs. SEAOC PV2-2017.
Underwriters Laboratories (UL). UL 1699B
Underwriters Laboratories (UL). Flat-Plate Photovoltaic Modules. ANSI/UL 1703.
UL 1741
4.3 Bibliography
ASTM International. Fire Tests of Roof Coverings. ASTM E108.
Grant, Casey C. Fire Fighter Safety and Emergency Response for Solar Power Systems. Fire Protection
Research Foundation (FPRF). May 2010 (revised October 2013).
International Standards Organization (ISO). General Requirements for the Competence of Testing and
Calibration Laboratories. ISO/IEC 17025:2005.
Jackson, P. “Target Roof PV Fire of 4-5-09, 9100 Rosedale Hwy, Bakersfield, CA.” City of Bakersfield,
California, Development Services/Building Department Memorandum.
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Los Angeles City Fire Department Requirement. Solar Photovoltaic System. FPB Requirement No. 96, 12/14.
Pagnamenta, R. “BP Solar Panel Blaze Raises Concerns Over Alternative Energy.” The Times, Monday June
29, 2009.
APPENDIX A GLOSSARY OF TERMS
Aerospace wind tunnel: A wind tunnel that simulates horizontal wind forces acting directly on an object. It
does not simulate conditions between the fans and the object within the lower portion of the boundary layer,
which is required to replicate the surface roughness exposure related to wind design of the building and
rooftop equipment. Neither does it replicate wind flow over a wall of a modeled structure below the rooftop
equipment that would be required to simulate actual suction effects in addition to the horizontal forces.
Allowable stress design (ASD): A structural design method in which the allowable stresses contain a safety
factor because the design is to stress levels that are only a percentage of the failure stresses.
Arc-fault circuit interrupter (AFCI): A device intended to provide protection from the effects of arc faults
by recognizing characteristics unique to arcing and by functioning to deenergize the circuit when an arc fault
is detected.
Arc prevention: Technology that uses advanced arc prevention techniques to prevent arcs from forming.
This exceeds the minimal requirements stated in NEC 2017 and UL 1699B, but these technologies are used
by some manufacturers of module level power electronics today. Limits low energy level (200 Joules).
Array size: The number of interconnected PV panels (the minimum number of panels within each row and
each column) and the gross plan area occupied within a given array. There is usually a slight (fraction of
an inch) separation between panels in the east-west direction and sufficient separation (depending on panel
slope) between rows to prevent shadowing.Wind tunnel or field model tests should replicate the minimum
array size required. Data for a larger array does not justify the design for a smaller array.
Automatic module-level DC shutdown: Systems that have built-in module-level power electronics and
safety features that deenergize the PV array at the module level. These systems may be automatically
triggered by a loss of grid power, high temperatures, ground faults, arc faults, faulty connectors, faulty wiring,
rodent damage, etc. This is sometimes referred to as “safe DC.”
Ballasted: Not adhered to the roof cover below, nor fastened to the roof deck or structure. Resistance to
wind loads is provided by the weight of the panels, mounting equipment, and any additional ballast. (Same
as “loose laid.”)
Boundary layer wind tunnel: A wind tunnel with a long transition between the fans and the object, and
that has obstructions to replicate the lower portion of the boundary layer and the surface roughness exposure
related to wind design of the building and rooftop equipment. Testing is done with scaled models of rooftop
equipment and the building upon which it is installed.
Closed mounting system: A PV mounting system that has a wind deflector on the high side (north side
in northern hemisphere and south side in southern hemisphere) of each row of panels, but may or may not
have one on the east and west ends of each row.
Coefficient of friction (µ): A dimensionless coefficient used to quantify resistance to lateral movement (in
this case, between the undersides of the panel mounts and the top surface of the roof cover). It is equal to the
lateral load resistance divided by the force normal to the two mating surfaces. This will vary depending on
the construction of the underside of the panel mount and the type of roof cover. Such construction includes,
but is not limited to, stainless steel, aluminum, coated metal, or metal with a pad (such as a piece of single-ply
roof cover material or rubber) adhered to its underside.
Computational fluid dynamics (CFD): A form of computer modeling that uses numerical methods and
algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the
calculations required to simulate the interaction of fluids with surfaces defined by boundary conditions.
Validation of such software is performed using a wind tunnel.
DC-to-DC converter: A device installed in the PV source circuit or PV output circuit that can provide an
output dc voltage and current at a higher or lower value than the input dc voltage and current.
DC-to-DC converter output circuit: Circuit conductors between the dc-to-dc converter source circuit(s)
and the inverter or dc utilization equipment.
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DC-to-DC converter source circuit: Circuits between dc-to-dc converters and from dc-to-dc converters to
the common connection point(s) of the dc system.
FM Approved: Products or services that have satisfied the criteria for Approval by FM Approvals. Refer to
RoofNav, an online resource of FM Approvals, for a complete list of roofing products and services that are FM
Approved.
Inverter: An electrical device used to convert direct current (DC) electrical power to alternating current (AC)
electrical power.
Load and resistance factor design: Also known as “strength design” or “ultimate design,” it is a structural
design method that provides a safety factor by applying factors to the loads, and in some cases a lesser
safety factor to the resistance of the materials.
Loose Laid: not adhered to the roof cover below, nor fastened to the roof deck or structure. Resistance to
wind loads is provided by the weight of the panels, mounting equipment, and any additional ballast. (Same
as “ballasted.”)
Moderate hail hazard area: Area in which the hail size does not exceed 1.75 in. (44 mm) for the 15-year
mean recurrence interval (MRI).
Non-sheltered PV panels: PV panels located on the exterior side of an array in the perimeter row(s) of
PV panels, and that are not sheltered from the wind load from other panels, and for which the wind load may
be greater than that of the interior, sheltered panels.
Open mounting system: A PV-mounting system that does not have a wind deflector on the high side (north
side in northern hemisphere and south side in southern hemisphere) of each row of panels.
Photovoltaic (PV) system: A system that uses solar panels to convert sunlight into electricity. It consists
of PV panels, support framework, and electrical connections and equipment to allow regulating and converting
the electrical output from DC to AC.
PV panel: An individual unit consisting of numerous cells, usually 60 or 72. It is usually about 39.4 in. (1
m) in the north-south direction and 65 to 77 in. (1.65 to 2.0 m) in the east-west direction. In most cases it
is bounded by edge framing. In some cases panels are also reffered to as modules, particularly for ballasted
situations. For anchored installations, three or four modules connected together may be considered a panel.
Rapid shutdown of PV systems on buildings: PV system circuits installed on or in buildings that include
a rapid shutdown function to reduce shock hazard for emergency responders.
Roof control joint: A construction joint that provides a break in the continuity of above-deck roof components
to prevent damage to the roof cover from thermal movement. This joint does not provide a break in the roof
deck.
Roof expansion joint: A construction joint that provides a break in the continuity of the building framing,
roof deck, and above-deck roof components to prevent damage to the building components from thermal
movement.
Setback: The distance between the outside edge of a roof supporting solar panels and the outer edge of
the solar array.
Severe hail hazard area: Area in which the hail size exceeds 1.75 in. (44 mm) but does not exceed 2 in.
(51 mm) for the 15-year mean recurrence interval (MRI).
Shadowing: Shade created by neighboring objects that necessitate relocation of solar panels and sometimes
openings within the array. This can create wind forces on solar panels immediately adjacent to the opening
that are higher than the forces on the interior of the array.
Sheltered PV panels: PV panels located on the interior side of the perimeter row(s) of PV panels that are
somewhat sheltered by the perimeter panels and for which the wind load is somewhat less than for the
perimeter panels.
Very severe hail hazard areas: Areas in the United States designated as such on the Hailstorm Hazard
Map in DS 1-34.
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APPENDIX B DOCUMENT REVISION HISTORY
The purpose of this appendix is to capture the changes that were made to this document each time it was
published. Please note that section numbers refer specifically to those in the version published on the date
shown (i.e., the section numbers are not always the same from version to version).
January 2021. Interim Revision. Minor editorial changes were made.
October 2020. Interim revision. Minor editorial changes were made.
July 2020. Interim revision. Minor editorial changes were made.
February 2020. Interim revision. The following changes were made:
A. Simplified the electrical recommendations section and added references to the 2017 edition of the
National Electrical Code.
B. Simplified wind design guidance for PV arrays that are parallel to and within 5 to 10 in. (125 to 250
mm) of the roof surface.
C. Expanded wind design guidance for sun-facing, sloped PV arrays.
October 2014. Interim revision. Added additional diagram (Fig. 12B, One-line example diagram to a PV
system with ground faults).
July 2014. This is the first publication of this document.
APPENDIX C SAMPLE PROBLEM: PV MODULES PARALLEL TO ROOF
The following example is based on ASCE 7-16, with slight modifications in accordance with SEAOC
PV2-2017.
C.1 Example
A proposed PV array is to be secured to a 24 in. (610 mm) wide, metal standing seam roof (SSR) using
extruded aluminum external seam clamps (ESC). The roof slope is ¼ in. per ft (1.2°). The PV modules will
be parallel to the roof surface. The distance between the flat part of the roof deck and the top edge of the
2 in. (50 mm) deep, integral aluminum frame of the PV module is to be 5 in. (127 mm). The PV modules are
60 cell and are 39 in. (1 m) wide and 66 in. (1.68 m) long. The long dimension of the PV modules will run
across the deck ribs. Three ESC will be used to secure each long edge of the PV module to the roof deck ribs
in accordance with Figure C.1-1. The horizontal space between modules will be 6 in. (152 mm) in their
longitudinal direction and 1 in. (52 mm) in the opposite direction. A minimum of 800 modules must be installed
to provide the required electrical output. The building is just slightly above sea level. Other details are as
follows:
H = 33 ft (10 m), Wind Exposure Category C, KZ = 1.0 per Table 3.8
WL = 246 ft (75 m), WS = 140 ft (42.7 m)
V = 110 mph (49 m/s) per Data Sheet 1-28
KZT = 1.0 KD = 0.85 per Data Sheet 1-28
Ke = 1.0 per Data Sheet 1-28
ESC must be installed at each deck rib to follow the wind load path of the SSR the PV modules are secured
to. The wind load path goes from the deck ribs to an internal clip, then through self-drilling screws securing
the internal clips into the top flange of steel purlins. The fire service requires a minimum 6 ft (1.8 m) wide
aisle every 100 ft (30.5 m). The goal is to minimize the wind load transferred to the existing roof.
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Deck
ribs
PV
66”
Standing
seam roof
N
6”
module
39”
1”
24”
9” 6”
9”
- External Seam Clamps (ESC)
Fig. C.1-1. Plan view of proposed layout for PV modules and clamps
C.2 Solution
STEP 1: Since the PV modules are parallel to the roof surface, and within 10 in. of the flat part of the roof
deck, per ASCE 7-16 and SEAOC PV2, the wind design load may be the same as for a low slope (≤7°) gable
roof. The value for GCP = is determined from Figure 30.3-2A of ASCE 7-16 and Data Sheet 1-28. (See Table
C.2.1)
STEP 2: One exception is that an edge factor (ϒE) = 1.5 must be applied to the exposed PV modules located
along each outer row closest to the roof edge and adjacent to aisles between arrays of all widths. Since
the largest area supported by any ESC is between 6.50 and 8.53 ft2 (0.6 and 0.8 m2) the GCP will be based
on an effective wind area (EWA) ≤ 10 ft2 (1 m2).
STEP 3: For an EWA ≤ 10 ft2 (1 m2), ϒA = 0.8 per ASCE 7-16 in this case. However, since the top edge of
the modules is ≤ 5 in. (127 mm) from the roof surface (h1 = h2), and the minimum gap (G) between modules
in each direction is ≥ ¾ in. (19 mm), SEAOC PV2 (2017) allows the design pressure to be further reduced
using a pressure equalization factor (ϒA) of 0.6.
qH = 0.00256KZKZTKDKeV2 = 0.00256 (1.0) (1.0) (0.85) (1.0) (110)2 = 26.3 psf
p = qH (GCP) ϒE ϒA
Zone 3:
p = (26.3) (-3.2) (1.5) (0.6) = -75.9 psf for the first row of exposed modules
p = (26.3) (-3.2) (1.0) (0.6) = -50.6 psf for the interior rows of modules
Zone 2:
p = (26.3) (-2.3) (1.5) (0.6) = -54.6 psf for the first row of exposed modules
p = (26.3) (-2.3) (1.0) (0.6) = -36.4 psf for the interior rows of modules
Zone 1:
p = (26.3) (-1.7) (1.5) (0.6) = -40.3 psf for the first row of exposed modules
p = (26.3) (-1.7) (1.0) (0.6) = -26.9 psf for the interior rows of modules
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Zone 1’:
p = (26.3) (-0.9) (1.5) (0.6) = -21.3 psf for the first row of exposed modules
p = (26.3) (-0.9) (1.0) (0.6) = -14.2 psf for the interior rows of modules
Allowable or design Wind pressures are summarized in Table C.3-2.
Table C.2-1. Values of GCp per ASCE 7-16 and Data Sheet 1-28
Roof Slope ≤7°
GCp per ASCE 7-16
Zone
1
2
1
1’
GCp
3.2
2.3
1.7
0.9
Roof Slope >7°, <= 20°
GCp per ASCE 7-16
3r
2n,2r,3e
1,2e
3.6
3
2.0
Roof Slope >20°, ≤27°
GCp per ASCE 7-16
3r
2n,2r,3e
1,2e
2.75
2.5
1.5
Roof Slope >27°, ≤45°
GCp per ASCE 7-16
3e
2n,3r
1,2e,2r
2.5
2
1.8
NOTE: All values of GCp are based on an effective wind area (EWA) of 10 ft2 (1 m2)
C.3 Summary
A. Wind design pressures shown in Table C.3.-2 should be used.
B. The following modules located in an outer row or column are considered “exposed” and should be designed
using the higher wind loads that include an edge factor = 1.5:
1. The north and south edges of Arrays 1 and 2.
2. The west edge of array 1 and the east edge of Array 2.
3. Also, the east edge of Array 1 and the west edge of Array 2 require an edge factor = 1.5.
C. The tempered glass for the proposed solar panel is 3.2 mm (1/8 in.) thick. Per ASTM E 1300, the allowable
wind pressure (short duration) is only 102 psf. A test of the PV module indicated that the aluminum frame
failed catastrophically at 105 psf ( 5.0 kPa). That is equivalent to an allowable load of only 65.6 psf ( 3.1 kPa)
with a safety factor of 1.6. Given that there is sufficient room on the roof, providing a minimum setback of
20 ft (6.1 m) for the PV modules installed per Figure C.3-2, is a preferred solution.
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19.8 ft
2
3
19.8 ft
19.8 ft
19.8 ft
3
6.6 ft
19.8 ft 19.8 ft
1
2
1’
2
1
3
3
2
Fig. C.3-1. Wind zones for low-slope roofs (<=7°) per ASCE 7-16
Roof edge
246’
40’
20’ Setback
1
1
Array 1
20’
Setback
N
20’ Setback
Array 2
1’
1’
1
1
1’
1’
140’
1
40’
1
6’
20’ Setback
- Zone 1 exposed
- Zone 1 sheltered
- Zone 1’ exposed
- Zone 1’ sheltered
Not to scale
Fig. C.3-2. Various wind zones for proposed PV array in the example
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Table C.3-2. Preliminary Wind Design Pressures
Zone
3
2
1
1’
GCP
-3.2
-2.3
-1.7
-0.9
ESC Location
Outer Edge/
Exposed
Shielded
Outer Edge/
Exposed
Shielded
Outer Edge/
Exposed
Shielded
Outer Edge/
Exposed
Shielded
Edge Factor
(E)
1.5
1.0
1.5
1.0
1.5
1.0
1.5
1.0
Ultimate
Zone
Allowable Wind
Resistance
Dimensions*, ft Uplift Pressure, with SF = 1.6
(m)
Psf (kPa)
for PV Modules
L-shaped, 6.6
-75.9 (3.6)
-121.4
(2.0)
perpendicularto
-50.6 (2.4)
- 80.9
roof by 19.8
(6.0) parallel to
roof
Between roof
-54.6 (2.6)
-87.4
edge, Zone 3
and a point
-36.4 (1.7)
-58.2
19.8 (6.0)
perpendicular
to roof edges
Between 19.8
-40.3 (1.9)
-64.5
(6.0) and 39.6
(12.1) in from
-26.9 (1.3)
-43.0
roof edges
Beyond 39.6
-21.3 (1.0)
-34.1
(12.1) in from
the roof edges
-14.2 (0.7)
-22.7
Ultimate
Resistance
with SF = 2.0
for Clamps
- 151.8
- 101.2
-109.2
-72.8
-80.6
-53.7
-42.6
-28.4
C.4 Discussion
Several options were considered to provide the required number of modules, but minimize wind forces applied
to the roof. Limiting the distance between the modules and the roof surface to 5 in. (635 mm), and providing
a minimum gap of ¾ in (19 mm). between modules provides a significant reduction in the wind uplift design
pressure as ϒA is reduced to 0.6. Note that this is allowed per SEAOC PV2-2017, but this value is limited to
0.8 in ASCE 7-16.
Another factor is the setback distance from the edge of the roof to the first row of PV modules, which often
is 10 ft (3.05 m) to 15 ft (4.6 m) on all 4 sides of the building. In reviewing Table C.3-1, it can be noted that
the wind pressure has been further reduced considerably by increasing the setback distance to 20 ft (6.1
m) on all sides and placing the modules in Zone 1 and 1’, and not in Zone 2 or 3.
As the local fire service required a minimum 6 ft (1.8 m) wide access aisles at maximum distances of 100
ft (30.5 m), the modules along each side of the aisle must use an edge factor = 1.5 since the aisle is > 4 ft
(1.2 m) wide.
This will still allow enough room for the required minimum of 800 modules by installing the modules in two
- approximate 100 by 100 ft (30.5 by 30.5 m) arrays. Within each array 14 modules in each of 30 rows are used
(see Figure C.3-2). This allows for up to 840 modules total. It also simplifies the allowable wind design, which
is now summarized in Table C.4-1.
Table C.4-1. Final Wind Design Pressures
Zone*
1
1’
GCP
-1.7
-0.9
ESC Location
Outer Edge/
Exposed
Shielded
Edge Factor (E)
1.5
Outer Edge
/Exposed
Shielded
1.5
1.0
1.0
Zone Dimensions,
ft (m)
Between 19.8
(6.0) and 39.6
(12.1) in from roof
edges
Beyond 39.6
(12.1) in from the
roof edges
* See Table C.3-2 for factored pressures.
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Allowable Wind
Uplift Pressure,
Psf (kPa) *
-40.3 (1.9)
-26.9 (1.3)
-21.3 (1.0)
-14.2 (0.7)